1. Field of the Invention
The present invention relates generally to the art of electric storage batteries, for example sealed automotive batteries. More particularly, the invention relates to vent caps for such batteries which provide a flow path for the escape of hydrogen and oxygen formed during the electrochemical reactions which take place in such batteries. Still more specifically, the invention relates to a vent cap which also controls the flow of electrolyte which may enter the vent cap to ensure that it is returned to the battery cells and does not become entrained in the flow of gases passing through the cap or flow through the cap to the gas exhaust port.
2. Description of the Prior Art and Technical Problems
Conventional lead-acid batteries, such as those used for automobiles, generally include a number of cells disposed in a battery housing. Each cell typically includes a plurality of positive and negative battery plates or electrodes, and separators are sandwiched between the plates to prevent shorting and undesirable electron flow during the reactions which take place during manufacture and use of the batteries. The plates and separators are immersed in a liquid electrolyte in the cells, the most common being aqueous sulfuric acid. The positive plate generally is constructed of a lead-alloy grid covered with lead oxide, while the negative plate generally contains lead as the active material, again covering a lead alloy grid.
In most battery constructions the battery housing includes a box-like base containing the cells and is made from a moldable resin. The housing is generally rectangular in horizontal cross-section, the cells being provided by vertical partitions within the housing. A cover is provided for the casing, the cover includes level terminal bushings and a series of fill holes to allow electrolyte to be added to the cells and to permit whatever servicing is required during use of the batteries. To prevent undesirable spillage of electrolyte from the fill holes, most prior batteries have included some sort of filler hole cap. Battery electrolyte spillage can be caused by a number of factors, including vibration or tilting as the vehicle with which this battery is used maneuvers during normal use. Electrolyte escape may also be caused by battery overheating, a problem especially pronounced in recent years with smaller car engines, which tend to run hotter than prior engines.
The electromotive potential of each battery cell is determined by the chemical composition of the electroactive substrates employed in the electrochemical reactions. For lead-acid batteries, such as those described above, the potential is usually about two volts per cell, regardless of cell volume. Vehicles manufactured by original equipment manufactures (OEM's) typically require twelve volt batteries, so most of today's batteries include six cells (6 cells.times.2 volts per cell=12 volts). The size of the housing for the battery is selected for the "envelope" for a particular vehicle, i.e. the physical dimensions defined by the vehicle manufacturer for containment of the battery in the engine compartment.
In addition to preventing spillage of electrolyte from the cells, the battery cover design and the filler caps need to perform an important and different function. This is because gases are liberated from lead-acid batteries during the charge and discharge reactions. Such reactions start at the time the battery is originally charged (called "formation") by the manufacturer or by the retailer or vehicle manufacturer. They also occur during normal operating use of the battery. Factors such as high current charge and discharge conditions, and changes in temperature, can affect the rate at which gas evolution occurs. The gas generation and evolution issues in lead-acid battery construction are particularly important because the gases are hydrogen and oxygen and it is important to vent such gases in a controlled way from the battery to prevent pressure buildups in the housing which could lead to electrolyte leaks, housing failures, or most significantly explosions within the housing. It is also desirable, and well known, to prevent an external flame from entering the battery through gas exhaust ports.
As will soon become apparent, many prior art devices are known for venting gases from battery cells in a manner which allows diffusion of the potentially explosive hydrogen gas. It will also become apparent that prior attempts provide vent caps with a flame or spark blocking material, generally known as an explosion attenuation element. However, it will also be seen that the focus of such prior art caps is on gas venting and the exhaust thereof through an explosion attenuation media.
The two problems previously mentioned, i.e. electrolyte spillage and gas evolution, are really interrelated and equally important in the construction of an effective vent cap system. For example, electrolyte may enter the vent cap through several mechanisms. One mechanism is through vibrational or tilting spray of electrolyte into the cap, and another is through a mechanism frequently referred to as pumping. The latter occurs when gas evolved in the battery bubbles from the cells and carries or forces electrolyte out the fill hole and into the cap. When electrolyte enters the caps of prior designs it may be carried out the exhaust passageway to cause damage to external battery components such as the battery terminals or adjacent engine components.
Original equipment manufacturers are beginning to recognize the importance of the dual function performed by vent caps and have instituted a number of testing specifications designed to ensure electrolyte retention in the cells. One such test involves tilting a battery 35.degree. about the longitudinal center line of the battery. This test is quite severe and could not be passed by a number of the prior art batteries using the vent constructions referred to below.
Early battery covers such as the "Gas Diffusion Device for Storage Batteries" described in U.S. Pat. No. 2,452,066, issued to Murphy on Oct. 26, 1948, used a diffusion member such as sheet asbestos or glass wool over the cells and supported by crossed grating bars. This simple design provided for gas diffusion, but not very effectively and it did not provide for practical electrolyte control.
In Hennen, U.S. Pat. No. 3,597,280, issued Aug. 3, 1971, a "Multiple Vent Plug Assembly" is described which includes three vent barrels entering three separate compartments, each of which is vented to the atmosphere. Circular baffles and other internal design features obstruct electrolyte from flowing to the vents.
In Hennen, U.S. Pat. No. 3,630,788, issued Dec. 28, 1971 entitled "Venting and Filling Device for Storage Batteries," a vent cap is described which uses an expandable funnel tube to permit filling of the cells with electrolyte, the lower end of the tube being immersed in the electrolyte. In this device, gases flow around the tube, through a tortuous path and through an expandable, porous diffusion material, such as a microporous material. If the gases are ignited outside of the microporous material, the cooling or heat dissipating effect of the microporous material will cause quick flame extinguishing and prevent flame propagation into the battery.
Evjen, et al. describe another system in U.S. Pat. No. 3,846,178, issued Nov. 5, 1974 and entitled "System for Absorption of Explosive Energy by Pressure Mitigation." In this system, a plurality of compressible, closed cell sheets are placed within the battery housing to keep the free space therein as small as possible. Since the volume of space within which explosive gases can accumulate is reduced, a large ratio of expansion is provided to maximize pressure reduction.
Another Hennen patent is U.S. Pat. No. 3,723,188 issued Mar. 27, 1973 for "Filling and Venting Device for a Storage Battery." In this device, a standpipe system ensures that electrolyte is maintained at a proper level in the cells. Gases percolate through a reservoir, forcing electrolyte to adjacent cells, and then are vented to the atmosphere through the hinged cap cover.
Another patent issued to the assignee of the present invention is Hennen's U.S. Pat. No. 3,879,227 entitled "Battery Vent Plug." This ganged plug features downwardly directed barrels for the fill holes and conical or sloping bottoms around drain opening which themselves include a slanted point to facilitate dripping of electrolyte into the cells. Gases follow a tortuous path through a porous diffuser adjacent the gas outlet. Semicircular baffles also surround each opening into the vent cap to facilitate directing electrolyte to the lowermost tip of the drain barrels. The gas pathway through the diffuser is upwardly. In one embodiment an open bottom tube is suspended from the top of the vent cap housing and depends downwardly over and is spaced above the cell vent opening.
Different explosion attenuation devices for single cells are disclosed in Melone, U.S. Pat. No. 3,915,753, issued Oct. 28, 1975 and entitled "Liquid Indicator for a Storage Battery with a Flame Barrier Vent Filter" and Auerbach, U.S. Pat. No. 3,944,437, issued Mar. 16, 1976 entitled "Explosion Proof Venting Device for Electrical Storage Batteries." Both provide tortuous flow paths for gases leaving the battery. The former additionally provides a liquid level indicator, while the latter provides a catalyst in the diffusion material to assist in the recombination of hydrogen and oxygen gases generated within the battery.
Another type of "Electric Storage Battery" is shown in Kitai, U.S. Pat. No. 4,117,205, issued Sep. 26, 1978. A gas cooling chamber separates electrolyte mist from discharged gas, the mist condensing to liquid electrolyte for return to the battery. Gases pass through a labyrinth flow chamber for being discharged through an air diffuser. The labyrinth also includes an electrolyte flow section.
Oxenreider, et al., in U.S. Pat. No. 4,278,742, issued Jul. 14, 1981 and entitled "Manifold Vented Battery Cover," also illustrates a battery cover employing a labyrinth design formed between two cover components which together form individual chambers for each battery cell, the chambers being interconnected by ports.
Another labyrinth system is disclosed in Ledjeff's U.S. Pat. No. 4,394,423, issued Jul. 19, 1983 and entitled "Closure Device for Lead-Acid Batteries." This device features an activated carbon gas diffusion system which in turn includes a wick for conducting condensed electrolyte back to the battery to replenish the liquid level.
A still further example of an explosion attenuation device is Jensen's U.S. Pat. No. 4,447,508, issued May 8, 1984 for "Reduced Maintenance Explosion Damage Resistant Storage Battery." A honeycomb material fills the space above the battery plates to subdivide the space into a plurality of small spaces and a sheet of catalytic recombination material is located above the honeycomb material to catalyze the reaction of any oxygen and hydrogen reaching that element.
In Greenlee, U.S. Pat. No. 4,463,069, issued Jul. 31, 1984 for "Battery Venting System", a porous flame arrestor is combined with an exhaust port, a combustion chamber and a buffer chamber between the combustion chamber and the arrestor. In Binder, et al., U.S. Pat. No. 4,751,154, issued Jun. 14, 1988 for "Battery Explosion Attenuation Material and Method", attenuation is provided by a porous compressible plastic material inserted in the head space of the battery.
Other explosion attenuation vent caps are described in commonly owned U.S. Pat. No. 4,916,034, issued Apr. 10, 1990 to Hulsebus, et al. and entitled "Battery Vent Strip." In this device, a vent cap includes a series of barrels with a strip extending transversely to the line of barrels, the strip including a porous explosion attenuation material. A plurality of channels couple the cells to the flame arrestor. Splash guards are provided to reduce electrolyte leakage into the exhaust flow path and the flame arresting material.
While a number of different solutions have been proposed in the aforementioned patents to the technical problems discussed earlier in this section of the specification, optimization has still not been achieved in one vent cap for the numerous problems with which the battery designer is faced--ensuring adequate electrolyte return, condensation, reducing electrolyte in the exhaust flow, pumping of electrolyte through the arrestor and tilting of the battery. All of these can result in electrolyte loss.
An improved vent cap for minimizing the possibility of electrolyte leakage from the battery, inhibiting the introduction of sparks or flame into the battery and efficiently directing gases from the battery is still needed. Such an improved vent cap would represent a substantial advance in this art.